1. Field of the Invention
The invention generally relates to a method and fixture for assembling supporting disk to a motor rotor, which is applicable to spindle motor for optical disk; and particularly relates to a method and fixture for assembling supporting disk to a motor rotor, through which the bias and concentricity are improved at one time.
2. Related Art
Accompanying with technical developments of mass data storage with optical disks, the precision requirements of spin motor for optical disk driver are getting higher and higher. In recent developments of HD-DVD, DVD-RAM and DVD-RW devices, for example, the axial bias and radial concentricity of the spin motor that carries optical disk have to be less than 0.015 mm and 0.03 mm respectively. An excellent performance and reliable data tracking of the disk driver relies on such a strict precision.
However, such precision requirements are very difficult for the current manufacturing process. FIGS. 1A and 1B illustrate a conventional spin motor and the method of measuring axial bias and radial concentricity. The spin motor 10 includes a stator 13, a rotor 12 and a supporting disk 111 mounted on the rotor 12. A plurality of jaws 111 are mounted in the central potion of the supporting disk 11 for grasping the center hole of an optical disk (not shown in the drawing) and driving the optical disk rotating along. In order to firmly grasp the optical disk, an anti-glide ring is usually formed concentrically around the jaws 111. The bias and concentricity of the spin motor 10 is checked as follows. Fixing the spin motor 10 on a base 14. Loading a measuring jig 15 on the supporting disk. The measuring jig 15 has a center hole 151 to be grasped by the jaws 111. Then, using micrometers 16a, 16b at top and circumference surfaces of the measuring jig 15 to measure the bias and eccentricity of the spin motor 10 by rotating the measuring jig 15.
The prior arts for improving the precision problems are usually by ways of improving component precision and adding supplementary processes during or after assembly. However, they still have difficulties and limitations as described below.
1) By Improving the Precision of Supporting Disk and Anti-glide Ring
Improving the precision of supporting disk is a straight and useful solution for improving the precision of spin motor. However, it is rather difficult to achieve the required bias and concentricity specifications by common process. Because the supporting disk has a complicated configuration that is usually made by plastic injection molding in order to lower the cost. The parameters of injection molding process, such as temperature, flow passage, thermal shrinkage and precision of mold, are complicated to control. Therefore, in order to meet the 0.03 mm concentricity requirement, the mold and the process have to be delicately modified and carefully controlled. The 0.015 mm bias requirement that depends on excellent molding and manufacturing techniques is further difficult to obtain.
Also, the 0.015 mm axial bias of the supporting plate requires thickness parallelity tolerance of the anti-glide ring being within 0.015 mm. It is not easy to achieve for rubber or silicone moldings that require specific molding and manufacturing attendance.
The aforesaid reasons make the component precision control difficult and expensive.
2) By Adding Supplementary Process During or After Assembly
It is possible to re-machine the assembly of rotor and supporting disk by lathing or grinding, or furnishing anti-glide material on surface of the supporting disk so as to improve the precision of the assembly. However, the post machining only improves the bias but does not help the concentricity. Further, the machining generates dusts that may cause trouble to the assembly. The furnishing of anti-glide material is less reliable because it cannot produce surface patterns that are preferred for a higher reliability.
Some manufacturers apply multiple anti-glide rings on the supporting disk. By using rings of different thickness, the bias can be reduced. However, the process is complicated and time-consuming. The process is also helpless to the concentricity.
In different applications of the supporting disks, there are also functional axis problems of columnar variance, vibration, angular shift, angular velocity, tangent velocity, acceleration and so on. However, these parameters, similar to aforesaid bias and concentricity, are caused by fixing and adjustment of the rotor and supporting disk and have bothered the manufactures for a long time.